In the US today, more than 80,000 people - men, women, and children - live up to 23 hours a day in tiny cells without natural light, air, or human contact. Many remain there for months, years, or even decades.

The UN's expert on torture considers more than 15 days in solitary confinement a human rights violation. The US is the only democratic nation that makes widespread use of long-term solitary confinement in its prisons, even for minor, nonviolent infractions as simple as having too many postage stamps.

Is solitary confinement torture? What effect does it have on the people who endure it?